On 13/02/2018 09:15, David Woodhouse wrote:>>> >>> - if (!data)>>> + if (!data && !spectre_v2_ibrs_all())>>> break;>> This should check the value of IBRS_ALL in the VM, not in the host.> No, it's host we want. If IBRS_ALL is set in the host, we set the> actual hardware MSR once at boot time and never touch it again. The> SPEC_CTRL MSR we expose to guests is purely a no-op fiction.> > If spectre_v2_ibrs_all() is true then KVM should *never* actually pass> through or touch the real MSR.

That would be nice but unfortunately it's not possible. :(

The VM might actually not have IBRS_ALL, as usual the reason ismigration compatibility. In that case, that no-op fiction would be veryslow because the VM will actually do a lot of SPEC_CTRL writes.

So the right logic is:

- if the VM has IBRS_ALL, pass through the MSR when it is zero andintercept writes when it is one (no writes should happen)

- if the VM doesn't have IBRS_ALL, do as we are doing now, independentof what the host spectre_v2_ibrs_all() setting is.